TRIPOLI, Lebanon — A roadside bomb packed with nuts and bolts exploded near a bus in this northern city yesterday, killing 18 soldiers and civilians in Lebanon’s deadliest bombing in more than three years.

Many suspected the attack was staged by an Al Qaeda-inspired group to avenge a 2007 military offensive. Some questioned whether it was an attempt to disrupt a trip by Lebanon’s president, Michel Suleiman, to Syria later in the day to patch up relations between the neighboring nations.

That visit brought a key agreement for the two countries to establish full diplomatic relations for the first time, a move that could ease tensions that have fueled Lebanon’s turmoil.

On Tuesday, the Lebanese parliament approved a new national unity government in Beirut that groups pro-Western factions and Syria’s ally, the Hezbollah militant group — another step that Lebanese hope will move the country past three years of political crisis.

But the attack at Lebanon’s second largest city cast a shadow over political progress.

The bomb, detonated by remote control, went off around 8 a.m. during rush hour just as the bus pulled to the curb to pick up passengers on a main street at Tripoli. It flung shrapnel through the bus and the nearby sidewalk crowded with people headed to work.

The bus — coming from Akkar, a region farther north where many military personnel live — was pockmarked with holes, its windows shattered. Soldiers and bystanders carried away the dozens of wounded on downtown Banks Street, which was littered with glass.

“I jumped out of my car and saw bodies in the streets,” the owner of the nearby Rivoli theater, Nabil Sebaei, said. “Wherever I saw a body moving, I helped carry them to a car to drive them to the hospital.”

“There is no religion in the world that accepts such acts,” Mr. Sebaei said, visibly shaken.

Security officials told The Associated Press that 10 soldiers and eight civilians were killed and 46 people wounded, while a senior military officer said 15 died. The security officials, speaking on condition of anonymity because they weren’t authorized to speak with reporters, said several bodies had been torn to pieces, complicating the count.

The army described the blast as a “terrorist attack directly targeting the army.”

Suspicion quickly fell on Fatah Islam, an Al Qaeda-inspired Sunni extremist group that soldiers drove out of the nearby Palestinian Arab refugee camp, Nahr el-Bared, in a monthslong battle last year. The fighting killed hundreds of people and destroyed much of the camp.

Fatah Islam’s leader Shaker Youssef al-Absi, who is on the run, warned of vengeance in a January audiotape, saying his fighters would “hunt down the followers of Suleiman,” who was army commander during the fighting. Al-Absi and four Syrian militants were charged in March with a double bus bombing last year that killed three people.

However, pro-Syrian Parliament Speaker Nabih Berri said the timing of the explosion before Mr. Suleiman’s two-day visit to Damascus was meant to prevent “repairing the Lebanese-Syrian relations.”

Mr. Suleiman’s trip for talks with Syria’s president, Bashar Assad, was the first by a Lebanese president in three years.

During their talks, the two leaders agreed to establish full diplomatic relations — something the countries haven’t had since they both gained independence from France in the 1940s, Assad adviser Buthaina Shaaban said.

The agreement was a victory for Syria’s opponents at Lebanon, who said Damascus’ failure to allow formal ties showed it didn’t recognize Lebanon’s sovereignty and considered it part of Syria.

But Syria agreed to ties only after the formation of the unity government ensured that it maintains influence at Lebanon through its Hezbollah allies, which will have a veto over government decisions.

Syria dominated Lebanon since the 1970s, when its troops deployed in the country during the long Lebanese civil war. Its direct hold wasn’t broken until its troops were forced to withdraw in 2005 after an uproar over the assassination of Lebanese Prime Minister Rafik Hariri.

Syria’s opponents at Lebanon have accused it of seeking to maintain control through Hezbollah and contend it was behind a string of bombings, starting with the Hariri assassination, that killed well known anti-Syria figures. Syria denies any role.

Unlike the Tripoli bombing, most of those attacks have been targeted assassinations and not aimed at inflicting broader casualties. There had been no serious attacks on politicians or public places since February.

Syria’s Foreign Ministry denounced yesterday’s explosion and expressed support for Lebanon “against the hands that try to disrupt its security and stability.”

Lebanon’s stability largely rests now on whether its rival factions can get along in the national unity government. Since the Syrian withdrawal, the struggle for power between pro-Western forces led by Prime Minister Fuad Saniora and the pro-Syria side headed by Hezbollah has pushed Lebanon to the brink of a new civil war.

Mr. Saniora vowed that the Tripoli attack “will not affect the launching of our government” and said “Lebanon and the Lebanese will not kneel … or submit to the criminals and the terrorists.”

Tripoli, about 50 miles north of Beirut on the Mediterranean coast, has in recent weeks witnessed sectarian clashes between Sunni fighters and followers of the Alawite sect, an offshoot Shiite sect. Dozens of people have been killed or wounded.

Omar Karami, once a prime minister and a prominent Tripoli politician, said it was too early to know the motive in yesterday’s bombing, but said the attack could be linked to the 2007 Nahr el-Bared fighting since it appeared to target the military.

Fatah Islam claimed responsibility for a bomb that killed a soldier in Abdeh, near Tripoli, on May 31. Last Friday, about 2,000 supporters of hard-line Islamic groups protested in Tripoli to demand the release of prisoners suspected of plotting or carrying out attacks.